Transmission methods of the above mentioned general type forming the general field of this invention, between one or more base stations or reader devices and one or more transponders, are typically used for contactless identification systems, or particularly so-called radio frequency identification (RFID) systems, for example. It is also possible to integrate one or more sensors, for example for carrying out a temperature measurement, on the transponder. Such transponders are referred to as remote sensors. In this context, the transponder or remote sensor transmits, or especially backscatters, identification information or sensor information in a contactless manner from the transponder or sensor to the base station or reader device. Throughout this specification, the general term “transponder” should be understood to cover both passive and semi-passive transponders, as well as remote sensors that include sensor elements integrated in or connected to a transponder.
Such transponders, or particularly the receiving/backscattering or receiving/transmitting circuit arrangements thereof, typically do not include an active transmitter for actively transmitting the relevant data to the base station. Instead, the transponders are non-active systems that can be designated as passive systems if they do not comprise their own power supply, or as semi-passive systems if they do comprise their own power supply. Especially passive transponders derive the energy necessary for their operation from the electromagnetic field emitted by the base station.
In such non-active systems, the data transmission in the distant or far field of the base station using UHF waves or microwaves generally uses a so-called backscattering or backscatter-coupling between the transponder and the base station. In this regard, the base station emits electromagnetic carrier waves, which are modulated by the receiving/backscattering arrangement of the transponder according to a prescribed modulation process in accordance with the data that are to be transmitted from the transponder to the base station. With this modulation, the waves are then reflected or backscattered from the transponder back to the base station. The typical modulation processes used in this context include amplitude modulation, phase modulation, and amplitude shift keying (ASK) subcarrier modulation in which the frequency or the phase position of the subcarrier is varied.
The prior German Patent Applications DE 102 04 347 and DE 101 38 217 A1 (and its counterpart US Application Publication 2003/0133435 A1, the disclosure of which is incorporated herein by reference) disclose methods for carrying out a wireless data transmission between a base station and a transponder, in which data packets to be transmitted each comprise a header section, a data section including the useful data to be transmitted, and an end section. The useful data to be transmitted are encoded and transmitted through the use of suitable defined symbols that are identified in the header section of the data packet.
In this regard, a symbol serves for the definition or interpretation of the value of a character in the character sequence representing or embodying the data being transmitted. Such a symbol is typically represented or defined in connection with a time duration between two successive field gaps or so-called “notches” in the header section of the data packet. Such a field notch can be achieved or generated, for example in connection with an amplitude modulation, by the suppressing or damping of the carrier signal. Alternatively, as another example in the context of a double or dual sideband modulation, such a field notch can be generated by switching over the phase position of the carrier signal. The transponder decodes the received data packets on the basis of the symbols contained and defined in the header section, more particularly in connection with the time durations that have been respectively allocated to the respective symbols. For determining the value of a respective character, the transponder compares the determined time duration of the respective character with the time durations of the symbols as defined in the header section of the data packet.
In the data transmission from the transponder to the base station, in some cases it is provided that each character or bit that is to be transmitted is transmitted synchronously with a clock signal or clocking pulse prescribed by the base station. In this context, the modulation state is fixed and/or known at the beginning of the time interval, and then a change-over of the modulation state is carried out after the elapse of the time duration of the symbol allocated in the header section to the character or bit. As an alternative thereto, the data transmission is typically carried out a synchronously.
Through the selection of the time intervals or particularly the time durations respectively allocated to the respective symbols, it is possible to adapt the transmission rate to the prevailing transmission conditions within a certain transmission rate range. This range of the transmission rate is limited, among other things, by the point at which the transponder, or rather particularly the encoding/decoding unit provided in the transponder for this purpose, can no longer resolve or distinguish the time differences between various different time durations that are respectively allocated to the symbols or characters. In this regard, a higher time resolution generally goes along with a higher current consumption of the transponder, for example because it is necessary to increase the clock frequency of a counter used for the time duration determination, or the charging current of a functionally corresponding analog RC-stage in the encoding/decoding unit. Because a rather low power density prevails in the far field of the electromagnetic waves emitted by the base station, and this power density serves for the power supply of the transponder (especially the passive transponder), the achievable transmission distance or range is reduced as the current consumption of the transponder increases. The parameter or parameters determining the encoding or the decoding, or especially the encoding/decoding unit, is thus typically statically configured in such a manner so as to achieve a sufficient compromise between the capability of a high time resolution and conditional thereon also a high achievable transmission rate on the one hand, and a low current consumption on the other hand.